The scaling down of the metal oxide semiconductor field-effect transisto r (MOSFET) has fostered the development of new characterization techniqu es that must be able to probe features of ever smaller dimensions. One o f the key elements is the control of the properties of the ultra-shallow junctions (USJs) encountered in the source and drain extension regions of a MOSFE T. In this thesis, we have developed the theory of photo modulated optical reflectance (PMOR) for the characterization of USJs in silicon. We have assessed the theory by comparing it with experimental measurements on Bo ron doped chemical vapour deposition box-like profiles acquired with the Carrier Illumination (CI) metrology tool. CI allows to measure the prob e laser differential reflectance as a function of the power of the pump laser, also known as a power curve. The possibilities and lim itations of PMOR and especially of PMOR on CI have been deeply assessed. The work has been divided into two main tasks, namely the direct and the inverse problem. The direct problem, i.e. the simulation of a power cur ve from a known active doping profile, has been addressed through the de velopment of a finite element code for the simulation of a semiconductor under optical injection, of suitable approximations and of compact expr essions for speed optimization as needed for solving the inverse problem . The inverse problem, i.e. the reconstruction of the active doping profile from a given power curve, has been addressed using different methods of increasing complexi ties, including direct nonlinear optimization based on iterations on the direct problem. We have shown that CI was able to reconstruct box-like doping profiles w ith junction depths in the range 15-70 nm and with active doping concent rations of up to 1e20 /cm3. The accuracy of the technique is however str ongly affected by surface recombinations, which limits its practical use in the present implementation. We believe, however, that this limitatio n could be eliminated by using an ultrafast (sub-picosecond) pumping mec hanism, and we have proposed a reconstruction method that would be suite d for the reconstruction of arbitrary monotonic non-retrograde doping pr ofiles.